In response to biotic and abiotic stress, plants activate inducible defense mechanisms. Secondary metabolites that may be toxic to attacking herbivores and pathogens, and protect plant tissues from abiotic stresses, are a common component of induced defenses.
Poplar, for example cottonwoods, poplars, and aspens, hereafter referred to collectively as poplar (Populus spp.), has become an important perennial plant. The defense-related phenylpropanoid metabolism of these ecologically important trees is complex. The major defense phenylpropanoids produced in poplar leaves are the flavonoid-derived proanthocyanidins (PAs) and the salicin-based phenolic glycosides (PGs).
PGs and PAs are the most abundant foliar phenolic metabolites in poplar, and together can constitute more than 30% leaf dry weight. Salicin-based PGs are constitutively produced in poplar leaves and function as potent anti-insect herbivore compounds. Although not often rapidly induced by herbivory, PG levels can exhibit considerable genotypic variability and are also influenced by environmental factors such as light and nutrient availability.
PAs are constitutively produced in poplar leaves however; in some genotypes concentrations rapidly increase in response to stress treatments for example insect herbivore feeding, mechanical wounding, defoliation, pathogen infection, and exogenous application of jasmonic acid. PA accumulation following wounding and herbivore attack generally occurs both locally at the site of damage, and systemically in distal leaves. Leaf PA levels are also strongly influenced by environmental conditions. Nutrient limitation and high light levels have been linked to greater PA. Increased PA levels have also been reported in P. tremuloides grown under elevated ozone.
Rapid stress-induced production of PAs in poplar leaves typically follows the transcriptional activation of the biosynthetic pathway. The strong activation of the PA biosynthetic pathway following insect herbivore damage suggests that these compounds function in herbivore defense. However, despite being rapidly induced by insect herbivores, experimental evidence indicates that unlike PGs, PAs are not strong, broad-spectrum anti-herbivore compounds.
Regulation of PA biosynthesis has been characterized in Arabidopsis, where TT2 regulates PA production specifically in the seed testa, in a tissue dependent manner. Regulation of PA production in Arabidopsis seed testa involves biosynthetic gene activation by a MYB-bHLH-WDR complex composed of the TT2, TT8, and TTG1 proteins. The R2R3MYB protein TT2 confers target gene specificity to the complex, leading to the activation of genes from the late flavonoid pathway for PA biosynthesis, for example BAN (ANR), TT12, and AHA10. AHA10 is a vacuolar ATPase, which is required for PA accumulation in Arabidopsis seed coats to energize transport via the tt-12 MATE transporter.
It was shown that TT2 does not regulate the early flavonoid biosynthetic genes, and therefore does not regulate PA biosynthesis independently. TT2 must work in collaboration with other genes. Further, TT2 activity is not correlated with an accumulation of PA and overexpression of TT2 alone does not lead to the accumulation of PAs.
A second PA-specific MYB gene, VvMYBPA1, isolated from grapevine (Vitis vinifera) was found to regulate both PA-specific structural genes of the late flavonoid pathway, and early flavonoid structural genes, for example those encoding chalcone synthase (CHS) and chalcone isomerase (CHI). Despite the established role of R2R3 MYB proteins in the developmental regulation of PA biosynthesis, a protein that regulates expression of all of the biosynthetic structural genes and regulates accumulation of PA, has yet to be disclosed.
It is an object of the present invention to overcome the deficiencies in the prior art.